Known methods of treating metal surfaces to improve paint adhesion and corrosion resistance of painted metal surfaces include two general classes of chemistries. The first class is based on traditional conversion coating types of chemistries, such as zinc phosphate, iron phosphates, chromium chromate, chromium phosphate, etc. The second class is based on more recent developments in the metal pretreatment industry and is characterized by what is now referred to as “dried-in-place” technology. Traditional conversion coating chemistries require rinsing of the metal substrate to remove applied pretreatment solution. Dried-in-place chemistries allow for the applied solutions to be dried on the metal substrate to which they are applied, without rinsing prior to the application of a paint.
Chromium compounds have been used as traditional conversion coatings to treat metal surfaces. Hexavalent chromium compounds show toxicological effects and have been determined by the Environmental Protection Agency to be a risk to the environment and by the Occupational Safety and Health Agency to be a health risk. Moreover, hexavalent chromium compounds such as chromic acid are used in some of these systems and are classified as carcinogens by these agencies.
As a result, much effort has been expended in developing compositions and methods for producing chromium-free pretreatments. For example, the owner of the present application has commercialized products that are successful in improving the corrosion resistance and paint adhesion of metal surfaces. Such compositions are disclosed in, for example, U.S. Pat. No. 5,859,106 to Jones et al, directed to an aqueous composition including a polymer system having carboxylic functional groups and hydroxyl groups and a compound of a group IV-B element.
Zinc phosphate coating solutions are commonly known in the art to pre-treat metal surfaces, such as cold-rolled steel, prior to sealing and/or painting. As known to one having ordinary skill in the art, zinc phosphate coatings have been used for years to improve the adhesion of paint and other finishes to metal surfaces and to achieve corrosion resistance with these finishes. Some proprietary oil and wax formulations now on the market are capable of withstanding up to 240 hours Neutral Salt Spray (according to ASTM B117). Studies have shown that zinc phosphate coatings isolate corrosion sites and that the morphology of the zinc phosphate provides physical anchoring sites for sealing and/or painting solutions, which may be applied to the surface in subsequent processing steps. However, zinc phosphate solutions are not without their drawbacks. The drawbacks associated with the application of a zinc phosphate coating solution include the additional process steps which are required in its application, such as rinsing and heating the surface after application of the zinc phosphate solution. These additional steps require a considerable amount of time and energy consumption. Additionally, zinc phosphate processes are not environmentally-friendly as they contain phosphates and heavy metals, utilize large quantities of water for rinsing, and often cause high amounts of sludging in the process which causes solid and liquid waste disposal issues.